Author: isonic-content

Key points

• Bearers are the heavier beams that sit on the stumps, piers or posts. Joists run across the bearers at right angles and carry the floor.
• Load travels from the floor to the joists, from the joists to the bearers, and from the bearers down to the footings.
• Timber bearer and joist sizes for houses come from the span tables in the AS 1684 series or from the project engineer.
• Joist spacing is set by how far the floor material can span, which is why the floor sheet chosen changes the frame underneath it.
• Platinum “R” Floor has a recommended maximum of 800mm joist centres, compared with 450mm for 19mm yellow tongue particleboard and 600mm for 22mm red tongue.
• Platinum “R” Floor is fixed over the joists and works with the framing rather than replacing it.

Plenty of new Australian homes are built on a concrete slab, but many sites still call for a floor built above the ground. Sloping blocks, flood-prone land, reactive soils and highset Queensland homes all rely on a framed subfloor, and that frame is made of bearers and joists. Platinum Timber & Ply supplies the floor panel that sits on that frame, so this is how the frame looks from the builder’s side of the job, and how the floor on top changes what the frame has to do.

What Are Bearers and Joists?

Bearers and joists are the two layers of horizontal framing in a raised floor. Bearers are the heavier beams that sit on the stumps, piers or posts, and joists are the lighter members that run across the top of the bearers at right angles and carry the floor itself. Together they form the subfloor frame, the grid that collects the weight of the floor and everything on it and passes it down to the footings.

The simplest way to remember the difference is the order they go in. Bearers go on the supports, joists go on the bearers, and the floor goes on the joists.

How a Subfloor Frame Carries the Load

Each layer of the frame spans between the layer beneath it, so the load steps down through the structure in order:

1. The floor takes the weight of people, furniture and internal walls and spreads it to the joists.
2. The joists carry that load across the gap between bearers.
3. The bearers collect the load from every joist resting on them and carry it between the supports.
4. The stumps, piers or posts take the load down to the footings and into the ground.

Because each layer spans between the members below it, the layers are linked. Wider joist spacing asks more of the floor sheet, and a longer bearer span asks more of the bearer, which is why the parts that make up a subfloor, from footings to floor sheet are designed as one system rather than chosen piece by piece.

Bearers vs Joists at a Glance

	Bearers	Joists
Sit on	Stumps, piers or posts	The bearers, usually on top and sometimes fixed to the sides
Direction	Run across the line of supports	Run at right angles to the bearers, parallel to each other
Carry	The joists, and the load passed down through them	The floor sheets or boards directly
Common materials	Sawn hardwood, treated pine, engineered timber such as LVL, or steel	Treated pine, engineered timber or light-gauge steel
Size set by	Span between supports, the load carried and the material grade	Span between bearers, joist spacing, the load and the material grade

What Decides Bearer and Joist Sizes

For timber framing in houses, bearer and joist sizes are normally taken from the span tables in the AS 1684 series for residential timber-framed construction, or set by the project engineer. The tables account for the span, the spacing, the stress grade of the timber, the loads being carried and the wind classification of the site, which is why two houses on the same street can end up with different framing.

Steel subfloor framing is sized from the manufacturer’s design data or by the engineer, and panel floors laid over steel joists are fixed and finished in the same way as over timber.

Span tables aren’t reproduced here, because the right figures depend on the timber grade, the loads and the site’s wind classification. The current standard or the engineer’s design is the source to use for any specific frame.

Joist Spacing Is Set by the Floor on Top

Joist spacing is measured from the centre of one joist to the centre of the next, and it is decided by how far the floor material can span between supports. A thinner or less stiff floor sheet needs joists closer together, while a stiffer one can bridge a wider gap. The frame is planned around the floor, not the other way round.

Floor material	Published maximum joist spacing
19mm yellow tongue particleboard	450mm
22mm red tongue particleboard	600mm
Platinum “R” Floor	800mm (recommended maximum)

Particleboard figures are the manufacturer’s published limits for general residential flooring. The Platinum “R” Floor figure is from its published specifications. Actual spacing is always confirmed by the project engineer.

The difference shows up in the joist count. On a floor 7.2 metres long, joists at 450mm centres means 17 joists, at 600mm centres it means 13, and at 800mm centres it means 10. The engineer still sizes the joists for the spacing chosen, so this is an illustration of the count rather than a design, but fewer joists means fewer members to cut, lift, level and fix.

The same comparison runs through the rest of the floor. How Platinum “R” Floor compares with yellow tongue and particleboard on insulation and installation stages is part of the same decision, along with the main floor sheet materials set side by side.

Where the Floor Panel Sits on the Frame

A panel floor is laid over the joists as a platform, with each sheet fixed down to the joists beneath it. Platinum “R” Floor works the same way: it is fixed over timber or steel joists and doesn’t replace the bearers or joists. What changes is what arrives on the frame, because the panel combines the structural floor and the insulation, so the floor is insulated as soon as it is laid, with published values from R1.2 to R2.5.

The panel’s framing guidance is a recommended maximum of 800mm joist centres, a minimum joist thickness of 45mm and blocking under any load-bearing wall, with spans and load cases confirmed by the project engineer. The panel’s published specifications set out sizes, thicknesses and R-values, and the installation sequence on site follows the same order as any platform floor.

The panel also changes one of the fiddlier framing jobs on a raised floor. Wet areas normally need joists notched, dropped or run at a different height to form a set-down for the tiles. With Platinum “R” Floor, a strip of fibre cement flashing between the panel edge and the 19mm wet-area flooring creates the step down, so the joists under bathrooms and laundries stay at one height.

Bearers and Joists on Sloping and Elevated Sites

A framed subfloor is what makes a level floor possible on a slope. The stumps or piers are set to different heights, the bearers run level across them, and the joists and floor follow. The same frame lifts a home clear of flood-prone ground or reactive soil, and builders across Queensland use the suspended floor options a sloping block allows for exactly these sites.

A raised frame also leaves an open, ventilated space under the floor, and that space is where a floor loses heat. Whether that floor needs insulating comes down to the climate, the subfloor type and the energy assessment, and the answer for a floor over an open void is almost always yes.

A Frame Built Around a Better Floor

Bearers and joists do the same job they always have: they carry the floor and pass its load to the ground. What has changed is the floor that goes on top. A panel that spans further, arrives insulated and simplifies wet-area set-downs lets the frame be planned around a better floor rather than around the limits of a thin sheet. The frame stays the same. The floor does more.

To check joist spacing and panel thickness for a project, contact the Platinum Timber & Ply team with your plans.

Frequently Asked Questions

What is the difference between bearers and joists?

Bearers are the heavier beams that sit directly on the stumps, piers or posts, while joists are the lighter members that run across the bearers and carry the floor. Load travels from the floor to the joists, from the joists to the bearers, and from the bearers down to the footings. Platinum “R” Floor is fixed over the joists and works with both timber and steel framing.

Which way do joists run on bearers?

Joists run at right angles to the bearers and parallel to each other, usually sitting on top of the bearers and sometimes fixed to their sides. The floor sheets are then laid across the joists. Platinum “R” Floor panels are laid over the joists in the same way as conventional sheet flooring, so the framing layout doesn’t change.

How far apart should floor joists be?

Joist spacing is set by how far the flooring can span. Particleboard sheet flooring is published at a maximum of 450mm centres for 19mm yellow tongue and 600mm for 22mm red tongue. Platinum “R” Floor has a recommended maximum of 800mm joist centres with a minimum joist thickness of 45mm, and the project engineer confirms the final spacing for each design.

What size bearers and joists do I need?

For timber framing in houses, sizes come from the span tables in the AS 1684 series or from the project engineer, based on span, spacing, timber grade, loads and the site’s wind classification. Steel framing is sized from the manufacturer’s data or by the engineer. When the floor is Platinum “R” Floor, the joists should be at least 45mm thick, with spacing up to the recommended 800mm maximum where the engineer’s design allows.

Does Platinum “R” Floor replace bearers and joists?

No. Platinum “R” Floor is installed over engineered timber or steel joists and does not replace structural framing. It combines the structural floor and the insulation in one panel, which removes the separate underfloor insulation stage, and it removes the need to notch or drop joists to form wet-area set-downs.

Do bearers and joists need to be insulated?

The insulation belongs to the floor rather than to the framing. In a raised floor it sits between the joists, under the floor or inside the floor panel, and the level required depends on the climate zone, the subfloor type and the energy assessment. Platinum “R” Floor puts the insulation inside the panel, from R1.2 to R2.5, so the floor is insulated as soon as it is laid.

Key points

• In most cases, yes. A floor built over a ventilated void sits between a heated room and outside air, and without insulation it loses heat to the void.
• The National Construction Code sets floor insulation requirements by climate zone and treats open and enclosed subfloors differently, so the energy assessor or certifier confirms the target for each design.
• NCC 2022 lifted the energy efficiency standard for new homes to 7 stars, and that standard was kept when NCC 2025 was settled.
• Insulation and subfloor ventilation are separate jobs, and the insulation must never block the airflow.
• The easiest time to insulate a subfloor is while the framing is open, because access is the main reason underfloor insulation goes wrong later.
• Platinum “R” Floor combines the structural floor and the insulation in one panel, with published values from R1.2 to R2.5.

Energy requirements for new homes have tightened, and the floor is now part of the conversation in a way it rarely used to be. On a slab the question hardly comes up, but on a floor built above the ground, over stumps, piers or a framed subfloor, the space underneath is ventilated and effectively outdoors. Platinum Timber & Ply makes an insulated floor panel for exactly that situation, and this is a builder’s answer to the question of whether the floor needs insulating at all.

Should a Subfloor Be Insulated?

In most cases, yes. A floor built over a ventilated void separates a conditioned room from a space full of outside air, and without insulation the floor gives up heat to that air for as long as the room is warmer than the void. The right level of insulation isn’t the same everywhere, because it depends on the climate zone, on whether the subfloor is open or enclosed by perimeter walls, and on how the house is being assessed for energy compliance.

If the walls and ceiling are insulated and the floor is not, the floor becomes the weak point in the thermal envelope. Occupants notice it first as a floor that feels cold underfoot and a room that takes longer to warm than the rest of the insulation suggests it should.

Why a Floor Over a Void Loses Heat

Heat always moves from the warmer side of a building element to the cooler side. Under a raised floor the cooler side is a ventilated void, and that ventilation is deliberate, because moving air is the airflow a subfloor needs to stay dry. The void can’t simply be closed up to stop the heat loss.

That leaves the floor itself to do the work. Insulation between the room and the void slows heat flow in whichever direction the temperature difference runs, so the floor behaves like the rest of the envelope instead of acting as an open window underfoot.

The same layer affects sound. A floor spanning over an open space carries footfall more readily than one bearing on the ground, and the acoustic side of an insulated panel floor is decided by the same part of the build-up as its thermal performance.

What the National Construction Code Asks For

The National Construction Code treats the floor as part of a house’s building fabric. Its energy efficiency provisions for houses set floor insulation requirements by climate zone, and they treat a floor over an open, unenclosed space differently from a floor over a subfloor enclosed by perimeter walls. That is why the answer to “how much” changes from one site to the next.

There are two ways to show compliance. The elemental route follows the minimum insulation values for the climate zone directly, while a house energy rating (NatHERS) models the floor, walls, roof and glazing together. NCC 2022 lifted the energy efficiency standard for new homes to 7 stars, and Building Ministers kept that standard in place when NCC 2025 was settled, so the floor’s contribution to the rating is not going away.

Wherever insulation is used, it has to be installed properly to count. The code requires insulation to comply with AS/NZS 4859.1 and to form a continuous barrier, and subfloor wall insulation must not block ventilation openings. The energy assessor or certifier sets the target for each design, and the builder’s job is to deliver it on site without gaps.

When Subfloor Insulation Matters Most

Any floor over a void benefits from insulation, but some builds put more of the floor in contact with outside air than others:

• Elevated and highset homes, where the whole underside of the floor is open to the air and elevated and multi-storey builds leave the most floor area exposed.
• Sloping blocks, where one side of the floor can sit well above the ground and the suspended floor options that suit a slope create a deep, well-ventilated void.
• Cooler climates, such as Melbourne and regional Victoria and regional and alpine New South Wales, where heating demand is highest and a cold floor is felt for more of the year.
• Brisbane and South East Queensland, where elevated and sloping-block homes are common across Brisbane and the rest of Queensland, and the floor counts in the house’s energy rating like every other part of the envelope. That is where the energy savings a Brisbane home can make through its floor come from.
• Floors over open spaces, such as a room over a carport or an open undercroft, which is a raised floor in every practical sense.

Insulate During the Build, Not After

The easiest time to insulate a subfloor is while the framing is open. Once the floor is laid, the only way to add insulation is from underneath, working overhead in a crawl space, and that is where most underfloor insulation problems begin: gaps between batts, sagging, compression and pieces that fall away over time. The common underfloor methods and what goes wrong with each mostly come back to access.

Search data suggests how often floors are insulated after the fact. Google records around 3,600 searches a month in Australia for “underfloor insulation”, compared with around 390 for “subfloor insulation”, and the results for underfloor insulation are led by retail batts, boards and do-it-yourself installation guides. That reads as strong demand for fixing cold floors in homes that are already built.

For a builder, the subfloor gets one easy chance to be insulated properly, and that chance is during the build.

Building the Insulation Into the Floor

There are two broad ways to insulate a new raised floor. The first is to lay the floor sheets and add insulation between or under the joists as a separate stage. The second is to use a floor panel with the insulation already inside it, so the floor and the insulation go down together in one step.

Platinum “R” Floor takes the second approach. Each panel has a 17mm structural plywood top skin, a 7mm structural plywood bottom skin and an Australian-manufactured foam core, and it is fixed over the bearer and joist frame that carries the floor, so it works with the structural framing rather than replacing it.

Panel thickness	Core thickness	Panel R-value	Panel weight
54mm	30mm	R1.2	40.9 kg
64mm	40mm	R1.5	41.9 kg
84mm	60mm	R2.0	43.9 kg
99mm	75mm	R2.5	45.3 kg

Source: Platinum “R” Floor published specifications.

One detail in those figures is worth noticing. Moving from the thinnest panel to the thickest more than doubles the R-value, from R1.2 to R2.5, and adds only 4.4 kg to each panel, so a higher thermal target does not make the floor much harder to handle on site.

Because the insulation is part of the panel, it can’t sag, slip or be left out of a corner, and the result is consistent across the whole floor. Matching the panel thickness to the energy assessor’s target is the first step in specifying it, and the full panel range, sizes and R-values set out what is available.

Insulation and Ventilation Work Together

Insulating a subfloor never means closing it in. The ventilation that keeps the framing dry is a separate job, and the code is clear that subfloor wall insulation must not block ventilation openings. An insulated floor panel keeps the two jobs apart: the insulation sits in the floor, and the void underneath stays open to the airflow it needs.

A Floor That Is Insulated From Day One

For a floor built over a ventilated void, the answer is almost always yes, and the level is set by the climate, the subfloor type and the energy assessment. The more useful question for a builder is how to deliver it reliably, and building the insulation into the floor panel answers that before anyone has to crawl underneath. The insulation isn’t an extra stage. It’s already in the floor.

To talk through panel thickness for a project, contact the Platinum Timber & Ply team with your plans and request a quote.

Frequently Asked Questions

Is subfloor insulation required by the National Construction Code?

It depends on the climate zone, on whether the subfloor is open or enclosed, and on how the house is assessed. The NCC’s energy efficiency provisions for houses set floor insulation requirements by climate zone, and a NatHERS house energy rating can be used instead to show the whole design meets the 7-star standard. Platinum “R” Floor comes in four thicknesses from R1.2 to R2.5, so the panel can be matched to the target the energy assessor or certifier sets.

What R-value should a subfloor have?

There is no single figure, because the target depends on the climate zone, the subfloor type and the energy assessment for the house. The energy assessor or building certifier confirms the value for each design. Platinum “R” Floor panels are published at R1.2 (54mm), R1.5 (64mm), R2.0 (84mm) and R2.5 (99mm), which lets a builder choose the thickness that meets the design’s target.

Does subfloor insulation matter on an elevated home?

Yes. An elevated home leaves the whole underside of the floor exposed to moving outside air, so an uninsulated floor is the part of the envelope most affected by outdoor temperatures. Platinum Timber & Ply supplies Platinum “R” Floor to builders across Brisbane and South East Queensland, where elevated and sloping-block homes are common, and the insulation goes down with the floor in one step.

Can a subfloor be insulated after the house is built?

Yes, but only from underneath, which means working in the crawl space and fitting batts or boards between the joists overhead. Access is the main reason retrofitted underfloor insulation ends up gapped, compressed or sagging. Building with Platinum “R” Floor avoids the retrofit entirely, because the insulation is part of the floor panel from the day it is laid.

Does subfloor insulation stop the subfloor from breathing?

It shouldn’t. Ventilation and insulation are separate jobs, and the NCC requires that subfloor wall insulation does not block ventilation openings. With Platinum “R” Floor the insulation sits inside the floor panel, so the void underneath stays open to the airflow that keeps the framing dry.

Is subfloor insulation worth the cost?

Yes, when it goes in during the build, because the framing is open and nobody has to work overhead later. With Platinum “R” Floor the floor and the insulation are installed in one step, so the fair comparison is against two separate stages, floor sheets plus underfloor insulation, rather than against the price of a sheet on its own. The cost of an insulated panel floor is driven mainly by floor area, panel thickness and site access, which is why Platinum Timber & Ply quotes per project.